For a storage system to provide a host with volumes, there is a known practice that forms a hierarchically tiered pool from storage areas of multiple drives different in performance capability in the storage system and provisions the host with virtual volumes configured in the pool (for example, refer to PTL1, US 2011/0167236 A).
The storage system analyzes input/output (I/O) loads to the virtual volumes from the host and automatically relocates pages with high I/O load to a high-class tier composed of resources of expensive disks having high performance capability and pages with low I/O load to a low-class tier composed of resources of inexpensive disks having low performance capability (page relocation between tiers). This technique effectively satisfies performance request at lower total cost of ownership (TCO). US 2011/0167236 A also discloses a technique to control the number of pages allocated from SSDs so that the SSDs will not be a bottle neck.